1. Field of the Invention
This invention pertains to liquid electrostatic toner transfer stations and more particularly to a transfer station employing corona transfer capable of generating images having very high resolution.
2. Description of the Prior Art
In most instances of electrostatic imaging, the ultimate objective is to produce an image on a carrier sheet having the same appearance, texture and handling characteristics as any other image produced through the printing processes. To obtain this objective, an electrostatically created image is typically covered with toner particles and the toner particles are eventually transferred onto a sheet of paper in imagewise fashion, where they are affixed in a more or less permanent manner. Such electrostatic image producing or image reproducing apparatus is well known in the art. Also well known in the art are methods for transferring the toner particles onto a plain sheet of paper using either pressure rollers to which a bias potential may be applied, or electrical fields such as created by a corona wire to deposit charges on the back side of the paper and attract on its front surface charged toner particles from a toner developed image carrier surface.
The toner particles may be available either as a dry powder, or as a liquid suspension. An electrostatic imaging system, employing both toner particles in a liquid suspension and a corona transfer station is shown in U.S. Pat. No. 4,256,820 issued to Benzion Landa.
In the aforementioned patent, the transfer station is shown schematically as comprising a tack down roll which presses the transfer paper onto the toned surface of a photoconductive drum. The roll is followed by two corona wires in a shielded case. When a voltage is applied to the wires, toner particles are attracted from the surface of the photoconductive drum to the surface of the transfer paper. Paper separating means are available after the corona to pull the paper away from the drum.
While electrostatic copying has found great success, the electrostatic imaging process has shown limitations when applied to certain fields, such as in the printing industry. It is well known to mount on a drum a pre-exposed plate having imagewise conductive and non-conductive areas and through sequential toning and transfer to print multiple copies of the same image much as a printing press does. However, unless very slow printing speeds are employed, the high resolution needed in the printing industry has been beyond the capabilities of the electrostatic printing systems and apparatus.
Specifically, corona transfer resolution, as practiced heretofore, typically provides resolution of the order of 9 lines/mm. In contrast high quality proofing images used in the printing industry require a minimum resolution of 50 to 60 lines/mm or 1% to 99% dots in a 133 lines/inch screen. The term "lines/mm" is defined as a combination of lines and space, also sometimes referred to as line pairs/mm. A detailed description of resolution measurements will be found in pages 476-511 of "Electrophotography" by R. M. Schaffert, published by Focal Press, London, 1975. Definitions of screens, dots and percentage areas as used by the graphic arts industry can be found in "Principles of Color Proofing" by M. H. Bruno, published by GAMA Communications, 1986. While liquid toners tend to produce higher resolution images, corona transfer of thin toner layers showed cracking and mottling of shadow dots and solid areas. These defects degrade the image resolution to an unacceptable level. And while great improvements are obtained when slow speed, i.e., less than 0.5 inches per second, is used in the image transfer stage, such low speed is commercially unacceptable; a higher speed is required. A minimum of 2.2 inches per second is required for some commercially viable systems.